6.4 Other code in .asd files

Files containing defsystem forms
are regular Lisp files that are executed by load.
Consequently, you can put whatever Lisp code you like into these files.
However, it is recommended to keep such forms to a minimal,
and to instead define defsystem extensions
that you use with :defsystem-depends-on.

If however, you might insist on including code in the .asd file itself,
e.g., to examine and adjust the compile-time environment,
possibly adding appropriate features to *features*.
If so, here are some conventions we recommend you follow,
so that users can control certain details of execution
of the Lisp in .asd files:

Any informative output
(other than warnings and errors,
which are the condition system’s to dispose of)
should be sent to the standard CL stream *standard-output*,
so that users can easily control the disposition
of output from ASDF operations.